For many years, the electric horns commonly used on automotive vehicles have been of the type which generate sound by vibration of a diaphragm driven by an electromagnet motor. The horn typically comprises a housing with the diaphragm peripherally clamped thereto forming a motor chamber. The coil of the electromagnet is mounted within the chamber and a magnetic pole piece on the housing extends axially of the coil. A magnetic plunger on the diaphragm extends toward the pole piece for imparting motion to the diaphragm in response to periodic energization of the coil. The diaphragm provides a resilient suspension of the plunger for reciprocating motion relative to the coil; it has a spring characteristic whereby the diaphragm and the mass carried by it have a resonant frequency of mechanical vibration. The coil is energized from the vehicle battery through a mechanically actuated switch which is alternately opened and closed by movement of the plunger with the diaphragm. A vehicle horn of this kind is described in the Wilson et al U.S. Pat. No. 4,813,123 granted Mar. 21, 1989.
Vehicle horns of the type described above have been highly successful in meeting the needs of the automotive industry. However, it has been proposed to modify that type of horn by substituting an electronic solid state energizing circuit for the mechanical switching contacts. The mechanical switching contacts, in the horn described above, are operable by vibration of the diaphragm to alternately connect and disconnect the horn coil from the car battery so as to maintain the diaphragm in a state of vibration for generating the sound pressure waves of the horn. In the proposed use of an electronic solid state energizing circuit for the horn, the coil is energized from the car battery through an electronic switch which is alternately switched on and off by an electronically generated DC pulse train.
A vehicle horn which employs a solid state energizing circuit for the horn coil is disclosed and claimed in U.S. Pat. No. 5,049,853 to Y. S. Yoon granted Sep. 17, 1991 for "ELECTRIC HORN WITH SOLID STATE DRIVER" and assigned to the assignee of this application. In copending application Ser. No. 684,693 filed on Apr. 12, 1991 by Wilson et al, for "VEHICLE HORN WITH ELECTRONIC SOLID STATE ENERGIZING CIRCUIT" and assigned to the assignee of this application. The horn of application Ser. No. 684,693 has an energizing circuit in which the pulse repetition rate or frequency of the pulse train and the duty cycle of the pulse train are adjustable independently of each other. This permits setting of the pulse train frequency at a value which causes the diaphragm to vibrate at its resonant frequency and thereby obtain maximum sound pressure level output from the horn. It also permits adjustment of the pulse train duty cycle so as to set the amplitude of vibration of the diaphragm in relation to the impact or contact point between the plunger moving with the diaphragm and the fixed pole piece.
The vehicle horns of the type referred to above, with either mechanical switching contacts or electronic switching, are manufactured in two different sub-types. One sub-type commonly known as a "seashell" horn is provided with a resonant projector which generates sound by free vibration of the diaphragm. The resonant projector is a trumpet-like device comprising a spiral passageway which defines an air column of increasing cross-section from the inlet end at the diaphragm to the outlet end at a bell. A second sub-type of horn is commonly referred to as a "vibrator" horn. This horn is provided with a resonator which is a vibratory plate, usually of circular configuration, mounted at its center on the diaphragm and plunger. In this device, the horn is energized so that the plunger strikes the pole piece during each cycle of diaphragm motion and the force of the impact is transferred to the center of the resonator causing it to vibrate at its resonant frequency. The vibration of the resonator generates sound pressure waves which are propagated directly into the atmosphere without any intermediate coupling device.
The seashell horn and the vibrator horn produce distinctly different sounds. The vehicle is commonly provided with a pair of seashell horns or a pair of vibrator horns to produce a desired sound. One horn of each pair is designed for relatively low frequency and the other for high. For the vibrator horns this is typically 350 Hz and 450 Hz. For the seashell horns it is typically 400 and 500 Hz.
In such vehicle horns, it is desired to operate the horn so that the diaphragm is vibrated at its natural resonant frequency. This provides the maximum sound pressure level output from the horn for a given input power. Also, for the purpose of minimizing the power required to drive the horn, it is desired to have the air gap between the plunger and the pole piece at a minimum value consistent with the desired vibrational motion of the diaphragm. For a seashell horn, there is free vibrational motion of the diaphragm, i.e. without any physical contact of the plunger with the pole piece; on the other hand, in the vibrator horn, the vibrational motion of the diaphragm is limited by the impact of the plunger with the pole piece during each cycle of diaphragm vibration. To achieve this, the stroke length of the plunger must be correlated with the length of air gap which exists between the plunger and pole piece when the diaphragm is at rest.
It has been a common practice in the manufacture of vehicle horns of the type described above with an electromagnet driven diaphragm to set the air gap between the plunger and pole piece at a determined length, within manufacturing tolerances, during fabrication of the horn. After assembly, the horn is tested and, if necessary, certain adjustments are made. One of the tests, sometimes called the "buzz point" test is used to determine whether the horn produces a desired sound quality over the full range of voltage variation likely to be encountered in vehicle operation. In such horns provided with a mechanical switch contact, the voltage applied to the horn for this test is increased from a value below the rated voltage to a value higher than the rated voltage and the horn is checked audibly for a "buzz point" voltage. This buzz point voltage is that a which undesired striking of the plunger against the pole piece occurs. In the seashell horn no striking is desired and in the vibrator horn a striking with moderate force is desired. An adjusting screw is provided on the switch contacts and is adjustably positioned to increase or decrease the time duration of voltage applied to the horn coil. If the switch contacts can be adjusted so that the buzz point does not occur when the applied voltage is less than the upper limit of the specified operating range of voltage and, if the current drawn by the horn is not excessive, the horn is acceptable.
In the manufacture of horns with an electronic energizing circuit, as distinguished from mechanical switching contacts, the frequency and duty cycle of the energizing pulses applied to the horn coil must be set at values for each horn which will produce the desired performance in respect to sound pressure level and sound quality.
A general object of this invention is to provide a method and apparatus for adjusting the frequency and duty cycle of a horn having an electronic energizing circuit.